Drawing Out the Facts

The STEM Chicksmas Day Two: Expanding Universe

For the 12 Days of “The STEM Chicksmas” we’re highlighting 12 scientists who have contributed something innovative and exciting to their field. It is the season of giving, and these brilliant minds have given incredible gifts to the scientific community! This year we’re looking at 12 Nobel Prize winners from the past 15 years in the fields of Physics, Chemistry, and Physiology or Medicine.

Almost 14 billion years ago, the universe was infinitely dense and hot, and then it started expanding exponentially in what we call the Big Bang. Fast forward to today, and the universe is still expanding. Up until 1998, however, most scientists thought this expansion was slowing down. It’s a logical conclusion—something can’t expand forever, right? However, research done by Saul Perlmutter, Brian Schmidt, and Adam Riess proved otherwise.

When Einstein was devising his theory of general relativity, he was so troubled by the idea that gravity should eventually cause the universe to contract that he added a term called the “cosmological constant,” which would allow for a static universe. When Hubble made observations that led to the conclusion that the universe was expanding, Einstein considered adding the cosmological constant to be the “biggest blunder” of his life. For most of the 20th century, astronomers thought the cosmological constant was zero.

The experiments performed by the prize winners’ respective research groups were simple enough in theory—identify a distant supernova that is sufficiently and consistently bright and take two photographs of it three weeks apart. (This is of course much less simple in practice.) They were hoping to measure the rate at which the universe’s expansion was slowing down, so they graphed the supernovae’s brightness (dependent on distance) versus its redshift (dependent on the object’s velocity: a light source continually emits light as waves, and as the object moves away from the observer the waves expand and thus become “redder”). However, after three weeks the supernovae were actually less bright than expected, which meant they had moved further away than they should have. The best explanation for this phenomenon was if the expansion of the universe was accelerating rather than decelerating!

When we say the universe’s expansion is accelerating, we mean that the space itself between galaxies is actually increasing. Scientists aren’t sure what the cause of this is, so they refer to this unknown force as “dark energy.” One leading hypothesis is that space intrinsically has energy—in this theory, dark energy is represented by Einstein’s confusing cosmological constant. Quantum mechanical fluctuations could be the cause of this energy associated with empty space. However the prediction from quantum mechanics disagrees wildly with the observed value in the universe. This remains an unresolved problem in modern physics. Another confusing aspect of this dark energy is that it has only been the dominant force for the last 6 billion years—except for the time directly after the Big Bang, the universe’s expansion had been slowing down! Why that changed is unknown. One more astounding fact is that dark energy accounts for 68% of all of the universe’s mass-energy! Matter, like you, me and the Earth, makes up less than 5%.

The discovery that the universe’s expansion is accelerating rocked cosmology to its core. It found a place for the cosmological constant, something even Einstein couldn’t resolve, and it fundamentally changed our view of the universe. It also gives us a picture of what the fate of the universe might be—galaxies infinitely far apart from each other, and a cold and lonely place.